Light and scanning electron microscopy of the effects of Macrogyrodactylus congolensis (Prudhoe, 1957) on the skin of the African sharptooth catfish Clarias gariepinus (Burchell, 1822).

Clarias gariepinus (Burchell, 1822) is one of the two most actively cultured freshwater fish in Africa and therefore, economically important. Specimens of this species were purchased from a fish farm near Hartbeespoort Dam (North West, South Africa) and introduced into the tanks of the research aquarium in the Department of Zoology at the University of Johannesburg. However, the skin of these fish was infected with Macrogyrodactylus congolensis (Prudhoe, 1957), which proliferated profusely in the favourable conditions of the aquarium, posing a potential threat to its host. The current study was aimed at examining the pathology caused by M. congolensis on the skin of C. gariepinus. Species identification of the parasite was confirmed using light microscopy (LM), scanning electron microscopy (SEM) and DNA barcoding of the internal transcribed spacer region. Examination of the pathology was studied using LM of haematoxylin and eosin-stained sections (epoxy embedded) and SEM of parasites attached to the hosts' skin. Infected skin exhibited excessive mucus production, corroborated by an increased number of mucus cells alongside proliferated and abnormally enlarged club cells, resulting in varying thickness of the epidermal layer. At the site of attachment, the basement membrane detached from the dermis. Hamulus points and marginal hooks of the parasite pierce through the hosts' skin resulting in tearing. Epidermal cells and melanin granules were observed in the intestinal lumen of the parasite. Melanin granules were absorbed by the parasite's intestinal epithelium confirming that the parasite feeds on host skin tissue.

First isolation and scanning electron microscopy of haptoral sclerites of Macrogyrodactylus (Monogenea).

Macrogyrodactylus congolensis (Prudhoe, 1957) is one of six species of Macrogyrodactylus, all of which are endemic to Africa. This monogenean is a host-specific ectoparasite of the African sharptooth catfish, Clarias gariepinus (Burchell, 1822). It attaches to the host with a posterior haptor armed with sclerites. The specific morphology of sclerites is taxonomically significant and usually studied using light microscopy. The aim of the present study was to confirm the identification of macrogyrodactylid parasites using classic morphology (light microscopy of glycerine ammonium picrate mounted specimens) and molecular techniques (18S rDNA, ITS rDNA and cytochrome oxidase subunit 1 (COI) mtDNA). Additionally, the sclerites were accurately described with a technique not previously used for the genus, whereby haptoral sclerites were isolated by removing the encapsulating soft tissue with a digestion buffer and studied with scanning electron microscopy (SEM). Morphology and morphometry of studied specimens corresponded to available data for M. congolensis, confirming the identity of the parasite. All previous descriptions were summarized in a table and discrepancies discussed. Molecular analysis also confirmed the specimens to be M. congolensis, but ITS rDNA and COI mtDNA was more reliable than 18S rDNA in this regard. The isolation of haptoral sclerites and their study using SEM was successful, resolving the morphology of all sclerites. This study provided the first reconstruction of the haptor of a Macrogyrodactylus species following SEM analysis, as well as the first mtDNA for M. congolensis. Further study of isolated haptoral sclerites of other macrogyrodactylids is required to determine the full benefits of studying their isolated sclerites.